Bendix BP-1 User Manual

®

Bendix® BP-1™ Brake Proportioning Valves

SD-03-952

3/8 P.T.

SUPPLY PORT

1/8 P.T.

CONTROL PORT

3/8 P.T.

DELIVERY PORT

FIGURE 1 BP-1™ BRAKE PROPORTIONING VALVES

1.5

.340”

MOUNTING

HOLES (2)

PIECE NO.

STAMPED HERE

I.D. TAG STAMPED

BP-1 FRONT OR

BP-1 REAR

DESCRIPTION OF THE BP-1™ VALVES

The BP-1™ brake proportioning valves are incorporated into
the air brake systems to improve the controllability and
reduce the stopping distance of bobtail tractors during
braking. BP-1™, front and rear, valves reacting to the lack of
pressure in the trailer supply line reduce the braking effort
on the rear axles and increase the braking effort on the front
axle of tractors operating in the bobtail mode. Treadle
application force during bobtail operation resembles treadle
application force during normal operation with a connected
trailer.

A typical brake proportioning system is shown in Figure 6.
The system consists of a front and a rear brake proportioning
valve and a TR-3™ valve.

Caution: The BP-1™ front and BP-1™ rear valves are similar

in appearance but are NOT interchangeable. Note
identification tag attached to one of the cover
screws (Figure 1); it will be stamped ‘BP-1 Front’

or ‘BP-1 Rear’. The BP-1™ rear valve can be used
without installing a BP-1™ front valve, providing the
vehicle does not have a front axle ratio valve. If the
vehicle has a front axle ratio valve, the ratio valve
must be replaced with a BP-1™ front valve.

OPERATION

A tractor that is equipped with the BP-1™ rear brake
proportioning valve and TR-3™ valve only will respond in the
following manner. With the trailer supply valve pushed in
and a service brake application made, equal pressure (100%
of service application) will be delivered to each axle of the
tractor and trailer. When the trailer is disconnected from the
tractor and the trailer supply valve is pulled out, the front
axle of the tractor will receive 100% of the service brake
application pressure and the rear axle or axles approximately
25%. As application pressure increases, the differences
between the front axle and the rear axle(s) pressures
diminish.

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A tractor that is equipped with both a front and rear axle
brake proportioning valve with the trailer supply valve
activated (tractor/trailer mode) will experience 50% of service
brake application pressure on the front axle up to 40 psi.
With service applications above 40 psi the differential between
the supply and delivery diminishes until at 60 psi it is a
one-to-one ratio. The rear axles receive 100% of the service
application. With the trailer supply valve pulled out, (bobtail
mode) the front axle of the tractor will receive 100% of service
brake application and the rear axle(s) approximately 25%
during normal brake applications. As application pressure
increases, the differences between the front axle and rear
axle(s) pressures diminishes.

BP-1™ REAR VALVE (REFER TO FIGURE 4)

When the trailer supply valve is activated, air pressure is
delivered to the trailer via the supply line. At the same time
air is delivered to the TR-3
the air to the control port of the BP-1
application is made, air entering the supply port works upon
the total area of the upper and inner pistons and delivery
pressure equals supply pressure. See Figure 2, full delivery
mode.

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inversion valve which exhausts

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valve and a brake

FULL DELIVERY MODE

(NO CONTROL PRESSURE)

DELIVERY PRESSURE (PSI)

SUPPLY PRESSURE (PSI)

LIMITING MODE

(WITH CONTROL PRESSURE)

BLEND BACK

POINT

DELIVERY PRESSURE (PSI)

With the trailer supply valve released, air pressure in the
supply line to the trailer is exhausted to atmosphere. At the
same time the air pressure to the control port of the TR-3

FULL DELIVERY MODE

(NO CONTROL PRESSURE)

DELIVERY PRESSURE (PSI)

SUPPLY PRESSURE (PSI)

LIMITING MODE

(WITH CONTROL PRESSURE)

BLEND BACK

POINT

DELIVERY PRESSURE (PSI)

SUPPLY PRESSURE (PSI)

FIGURE 2 PERFORMANCE CURVE FOR NOMINAL

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BP-1

BRAKE PROPORTIONING REAR VALVE

SUPPLY PRESSURE (PSI)

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FIGURE 3 PERFORMANCE CURVE FOR NOMINAL
BP-1™ BRAKE PROPORTIONING FRONT VALVE

inversion valve is also exhausted. This sequence causes
full reservoir pressure to be delivered to the control port of
the BP-1™ rear valve. This air is contained on the underside
of the upper piston and when a brake application is made
the air entering the supply port of the BP-1™ rear valve can
only affect the surface of the lower and inner pistons. Due to
the reduced area of the lower and inner pistons, the valve
delivers approximately 25% of the supply pressure to the
relay valve. See Figure 2, limiting mode. In addition, spring
#8, Figure 4 causes the lower piston to be initially held open
until initial delivery pressure closes the lower piston and the
valve will limit normally. The purpose of this initial delivery is
to offset the relay valve crack pressure when the vehicle is
operating bobtailed and enhance the performance of the
proportioning system.

BP-1™ FRONT VALVE (REFER TO FIGURE 5)

When the trailer supply valve is activated air is supplied to
the trailer via the supply line at the same time air is delivered
to the TR-3™ inversion valve which exhausts the air to the
control port of the BP-1™ front brake proportioning valve.
With air pressure removed from the control port of the BP-1
valve and a brake application is made, air entering the supply
port works upon the total surface area of the upper and lower
piston. The upper piston is restricted by the piston spring
up to 40 psi; therefore, brake applications up to 40 psi can
only displace the lower piston, which due to its surface area

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